Talk:Damage processes

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Autoren: Michael Steiger

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Introduction

Salts play a central role in the weathering of porous building materials. Salt damage is always the result of phase transformations in the pore space and the associated crystal growth. In the pore space the enclosed, expanding crystals can build up pressures, that exceed the mechanical strength of many materials and thus cause their failure. Considerable progress in understanding the actual deterioration mechanism caused by crystal growth, has been achieved in recent years. Regardless of the mechanism, an exact knowledge of the conditions causing the unwanted phase transformation, is crucial for the development of suitable strategies to prevent the damage.

Phase transformation – Crystallization

The most important phase transformation process that can lead to damage in building materials, is the crystallization of a salt in the pore solution. For example, this process can be triggered by the evaporation of water or temperature fluctuations, because the solubility of many salts distinctly depends on temperature. The process becomes critical, when it occurs in cycles and under unfavorable conditions, ie when salts repeatedly dissolve and crystallize. Such cyclic crystallization processes occur when the humidity level of the material continuously fluctuates. High moisture supply, erg. irrigation, usually dissolves soluble salts in building materials, crystallization then occurs at subsequent drying. Condensation can also be a source responsible for humidity and the interim dissolution of salts.

Finally, the properties inherent to the salts, determine the moisture content of a building material. Especially the process of deliquescence is of particular importance. When the deliquescence relative humidity (DRH) is exceeded, the salt absorbs moisture from the ambient air and forms a solution. With a further increase in humidity, more water is absorbed and the solution becomes more diluted. Therefore, due to their hygroscopicity, salts can contribute significantly to the moisture penetration in masonry. If the relative humidity of the ambient air decreases to below the DRH, the salts crystallize. Consequently, just the fluctuation of relative humidity around the DRH can lead to cyclic crystallization processes and usually, after a short period of time, cause severe damage to the material.

Phase transformation – Hydration

Crystal growth inside the pores can also take place during hydration reactions. Because the phase in the higher stage of hydration has a lower density, hydration reactions increase the filling degree of the pores, resulting in the crystals of the hydrated stage of the salt, growing against the pore wall and the build up of hydration pressure. Under unfavorable condition, cyclic hydration reactions are also possible. The moisture needed for the hydration reaction can be introduced through precipitation or condensation. Also an increase in relative humidity can be sufficient to trigger the reaction.

Salts and indoor climate

Arnold and Zehnder [Arnold.etal:1991]Title: Monitoring Wall Paintings Affected by soluble Salts
Author: Arnold, Andreas; Zehnder, Konrad
have first investigated the properties of salts in connection with the previously described situations. They correlated their observations on constructions, with the properties of the different salts and the climatic conditions. Their findings showed that the dynamic of salt damage processes is mostly determined by the interaction between salt mixtures in the pore space and the ambient humidity, IE the conditions of the indoor climate. For instance the relative humidity varies in the heated indoor environment periodically over the year and during the heating season the indoor climate reaches very low relative humidity levels of 30-40%. The result is a cycle, where conditions fall above or below the deliquescence or hydration humidity levels of a variety of salts, inevitably causing damage processes, if these salts have accumulated in a building material and are exposed to such indoor climate conditions. Conversely, the controlover the indoor climate offers a means to safely eliminate damage processes caused by crystal growth, provided that there is a contamination of a specific salt [Price:2000]Title: An Expert Chemical Model for Determining the Environmental Conditions Needed to Prevent Salt Damage in Porous Materials, European Commission Research Report No 11, (Protection and Conservation of European Cultural Heritage)
, [Steiger:2005c]Title: Salts in Porous Materials: Thermodynamics of Phase Transitions, Modeling and Preventive Conservation
Author: Steiger, Michael
. This also opens up the possibility of preventive conservation measures.

If a salt that does not form any hydrates, erg. sodium chloride (NaCl, halite) is present, damage can be prevented if the relative humidity is always kept below the deliquescence of this specific salt. The salt will then crystallize only once and stay immobilized (in the absence of other water sources). If the relative humidity level is kept above the deliquescence of NaCl, crystallization will never take place, the salt will permanently remain a solute in the pore, without any further material damage. Depending on the kind of salt, its concentration in the building material and the relative humidity, an increased Materialfeuchten could be the result of such a measure.

Phase diagrams

The deliquescence relative humidity of many salts present in building materials varies to a great extent and covers the full relative humidity range. For sodium chloride, however, the deliquescence is almost independent from temperature and amounts to approx. 75% RH, simplifying the prediction for climatic conditions. When looking at other salts, the DRH depends significantly on temperature. If these salts occur in different hydrate stages, it can be very difficult to define a suitable indoor air climate. In these cases phase diagrams showing the stability ranges of the different phases as a function of temperature and RH, can be very useful.

Salt mixtures

In the presence of pure salts and with the particular phase diagram as a basis, it is always possible- through choice of the appropriate indoor air conditions- to prevent phase transformation, i.e. crystal growth. Unfortunately, pure salts are rarely present in building materials, but there are usually relatively complex compound mixtures. In most cases the compounds dealt with on building sites are chlorides, nitrates, sulfates and sodium carbonates. The behavior of salt mixtures is much more complicated than the behavior of pure salts and information can generally not be derived only from the properties of the salts present in the compound. For example, salt mixtures cannot be characterized by a single deliquescence relative humidity, but in dependence of the composition of the mixture there is a relative humidity range, wherein fluctuations lead to phase transformations and crystallization processes. Phase diagrams of salt mixtures are therefore more complex and the prediction of suitable climate conditions can usually only be made with appropriate models. Elsewhere salt mixtures are discussed in more detail.

Literatur

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SLeithaeuser 17:55, 5 February 2012 (CET)

A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.

Deliquescence Relativ Humidity

relative humidity